In the broadest sense, a photometer is any instrument used to measure illuminance or irradiance. As applied in industrial photometry, a "photometer" is the general term covering instruments for detecting:

* scattered light intensity
* absorption
* fluorescence

Most photometers are based on a photoresistor or photodiode. Either exhibits a change in electrical properties when exposed to light, which can be detected with a suitable electronic circuit.

Photon counting

Some photometers measure light by counting individual photons rather than incoming flux. The operating principals are the same but the results are given in units such as photons/cm² or photons·cm^-2·sr^-1 rather than W/cm² or W·cm^-2·sr^-1.

Due to their individual photon counting nature, these instruments are limited to observations where the irradiance is low. The irradiance is limited by the time resolution of its associated detector readout electronics. With current technology this is in the megahertz range. The maximum irradiance is also limited by the throughput and gain parameters of the detector itself.

In airborne and space-based remote sensing such photon counters are used at the upper reaches of the electromagnetic spectrum such as the X-ray to far ultraviolet. This is usually due to the lower radiant intensity of the objects being measured as well as the difficulty of measuring light at higher energies using its particle-like nature as compared the the wavelike nature of light at lower frequencies. Conversely, radiometers are typically used for remote sensing from the visible, infrared though radio frequency range.

See also

* Photodetector – A transducer capable of accepting an optical signal and producing an electrical signal containing the same information as in the optical signal. The two main types of semiconductor photodetectors are the photodiode (PD) and the avalanche photodiode (APD).

* Photomultiplier – A device capable of detecting single photons and multiplying the signal several magnitudes.

* Spectrophotometer – A device which measures a spectral reflectance curve of a colorant